How come these epoxides don't open up as trans? Aren't they still attacking from the back?

[510586]

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[FeralisExtremum]

I could be wrong, but from what I can tell, there's no actual cis/trans here. We don't have any double bonds.

 

[510586]

I could be wrong, but from what I can tell, there's no actual cis/trans here. We don't have any double bonds.

Oh it was written like that in the back of the text. Typically if we add a group it adds on the opposite side if there is stereochem though. Here is an example

 

[100%permwinner]

Cis/trans usually apply to double bonds. The products all have rotatable single bonds and no stereocenters.

 

[510586]

Cis/trans usually apply to double bonds. The products all have rotatable single bonds and no stereocenters.

oh ok but had there been a ring is my above picture correct? Like still attacking on the back / opposite side

 

[100%permwinner]

The reaction are examples of SN2, so the nucleophiles attack from the back and the products have inverted stereochemistry. But I do not see any stereocenters in the reactants and products.

 

[510586]

The reaction are examples of SN2, so the nucleophiles attack from the back and the products have inverted stereochemistry. But I do not see any stereocenters in the reactants and products.

Oh I meant the above picture in the reply to Feralis, not the original. I was just double checking if I had the correct idea of what was happening. But I think it is. Thanks!

 

[FeralisExtremum]

Oh it was written like that in the back of the text. Typically if we add a group it adds on the opposite side if there is stereochem though. Here is an example

Yep, backside attack on epoxides, you got it.

 

[DAT Destroyer]

View attachment 194012

It would attack from the opposite side, but there is no stereochemistry to show here. The reactions are fine, just basic nucleophilic attacks. Only when other substituents are placed on the epoxide, can one appreciate the stereochemistry. The better conformation would have been to place the groups on opposite sides.

Hope this helps.

Dr. Jim Romano